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09 - Alkynes - Wade 7th

2.
Chapter 9 2
Introduction
• Alkynes contain a triple bond.
• General formula is CnH2n-2.
• Two elements of unsaturation for each
triple bond.
• Some reactions resemble the reactions
of alkenes, like addition and oxidation.
• Some reactions are specific to alkynes.

3.
Chapter 9 3
Nomenclature: IUPAC
• Find the longest chain containing the
triple bond.
• Change -ane ending to -yne.
• Number the chain, starting at the end
closest to the triple bond.
• Give branches or other substituents a
number to locate their position.

4.
Chapter 9 4
Examples of Nomenclature
• All other functional groups, except ethers and
halides have a higher priority than alkynes.

6.
Chapter 9 6
Physical Properties
• Nonpolar, insoluble in water.
• Soluble in most organic solvents.
• Boiling points are similar to alkane of same
size.
• Less dense than water.
• Up to four carbons, gas at room
temperature.

7.
Chapter 9 7
Acetylene
• Acetylene is used in welding torches.
• In pure oxygen, temperature of flame
reaches 2800°C.
• It would violently decompose to its
elements, but the cylinder on the torch
contains crushed firebrick wet with
acetone to moderate it.

8.
Chapter 9 8
Synthesis of Acetylene
• Heat coke with lime in an electric furnace
to form calcium carbide.
• Then drip water on the calcium carbide:
coke lime
This reaction was used to produce light for miners’ lamps and for the stage.
C CaO3 + +CaC2 CO
H C C H Ca(OH)2CaC2 + 2 H2O +

9.
Chapter 9 9
Molecular Structure of Acetylene
• Triple-bonded carbons have sp hybrid orbitals.
• A sigma bond is formed between the carbons by
overlap of the sp orbitals.
• Sigma bonds to the hydrogens are formed by using
the second sp orbital.
• Since the sp orbitals are linear, acetylene will be a
linear molecule.

10.
Chapter 9 10
Overlap of the p Orbitals of
Acetylene
Each carbon in acetylene has two unhybridized p
orbitals with one nonbonded electron. It is the
overlap of the parallel p orbitals that form the triple
bond (2 pi orbitals).

11.
Chapter 9 11
Bond Lengths
• Triple bonds are shorter than double or single
bonds because of the two pi overlapping
orbitals.

13.
Chapter 9 13
Acidity of Alkynes
• Terminal alkynes, are more acidic than other
hydrocarbons due to the higher s character of
the sp hybridized carbon.
• Terminal alkynes can be deprotonated
quantitatively with strong bases such as
sodium amide (-
NH2).
• Hydroxide and alkoxide bases are not strong
enough to deprotonate the alkyne
quantitatively.

14.
Chapter 9 14
Formation of Acetylide Ions
• H+
can be removed from a terminal alkyne by
sodium amide, NaNH2.
• The acetylide ion is a strong nucleophile that
can easily do addition and substitution
reactions.

16.
Chapter 9 16
Acetylide Ions as Strong Bases
• Acetylide ions are also strong bases. If the
SN2 reactions is not possible, then an
elimination (E2) will occur.

17.
Chapter 9 17
Show how to synthesize 3-decyne from acetylene and any necessary alkyl halides.
Another name for 3-decyne is ethyl n-hexylacetylene. It can be made by adding an ethyl group and a
hexyl group to acetylene. This can be done in either order; we begin by adding the hexyl group.
Solved Problem 1
Solution

18.
Chapter 9 18
Addition to Carbonyl Compounds
• Nucleophiles can attack the carbonyl carbon
forming an alkoxide ion which on protonation
will form an alcohol.

22.
Chapter 9 22
Show how you would synthesize the following compound, beginning with acetylene and any necessary
additional reagents.
We need to add two groups to acetylene: an ethyl group and a six carbon aldehyde (to form the
secondary alcohol). If we formed the alcohol group first, the weakly acidic —OH group would
interfere with the alkylation by the ethyl group. Therefore, we should add the less reactive ethyl group
first, and add the alcohol group later in the synthesis.
The ethyl group is not acidic, and it does not interfere with the addition of the second group:
Solved Problem 2
Solution

23.
Chapter 9 23
Dehydrohalogenation Reaction
• Removal of two molecules of HX from a
vicinal or geminal dihalide produces an
alkyne.
• First step (-HX) is easy, forms vinyl
halide.
• Second step, removal of HX from the
vinyl halide requires very strong base
and high temperatures.

26.
Chapter 9 26
Addition Reactions
• Similar to addition to alkenes.
• Pi bond becomes two sigma bonds.
• Usually exothermic.
• One or two molecules may add.

27.
Chapter 9 27
Catalytic Hydrogenation of
Alkynes
• Two molecules of hydrogen can add across the triple
bond to form the corresponding alkane.
• A catalyst such as Pd, Pt, or Ni needs to be used for
the reaction to occur.
• Under these conditions the alkyne will be completely
reduced; the alkene intermediate cannot be isolated.

28.
Chapter 9 28
Hydrogenation with Lindlar’s
Catalyst
• The catalyst used for the hydrogenation reaction is partially
deactivated (poisoned), the reaction can be stopped after the
addition of only one mole of hydrogen.
• The catalyst used is commonly known as Lindlar's catalyst and
it is composed of powdered barium sulfate, coated with
palladium poisoned with quinoline.
• The reaction produces alkenes with cis stereochemistry.

29.
Chapter 9 29
Mechanism
• Both substrates, the hydrogen and the alkyne, have
to be adsorbed on the catalyst for the reaction to
occur.
• Once adsorbed, the hydrogens add to the same side
of the double bond (syn addition) giving the product a
cis stereochemistry.

30.
Chapter 9 30
Reduction of Alkynes with Metal
Ammonia
• To form a trans alkene, two hydrogens must
be added to the alkyne anti stereochemistry,
so this reduction is used to convert alkynes to
trans alkenes.

34.
Chapter 9 34
Addition of HX
• One mole of HCl, HBr, and HI add to alkynes to form
vinyl halides.
• If two moles of HX is added, product is a geminal
dihalide.
• The addition of HX is Markovnikov and will produce a
geminal dihalide.

35.
Chapter 9 35
Mechanism of Hydrogen Halide
Addition
• The triple bonds abstract a proton from the hydrogen
halide forming a vinyl cation.
• The proton adds to the least substituted carbon.
• The second step of the mechanism is the attack by
the halide.

36.
Chapter 9 36
Anti-Markovnikov Addition of
Hydrogen Bromide to Alkynes
• By using peroxides, hydrogen bromide can be added
to a terminal alkyne anti-Markovnikov.
• The bromide will attach to the least substituted
carbon giving a mixture of cis and trans isomers.

37.
Chapter 9 37
Hydration of Alkynes
• Mercuric sulfate in aqueous sulfuric
acid adds H—OH to one pi bond with a
Markovnikov orientation, forming a vinyl
alcohol (enol) that rearranges to a
ketone.
• Hydroboration–oxidation adds H—OH
with an anti-Markovnikov orientation,
and rearranges to an aldehyde.

38.
Chapter 9 38
Mercuric Ion Catalyzed Hydration
of Alkynes
• Water can be added across the triple bond in a reaction
analogous to the oxymercuration–demercuration of
alkenes.
• The hydration is catalyzed by the mercuric ion.
• In a typical reaction, a mixture of mercuric acetate in
aqueous sulfuric acid is used.
• The addition produces an intermediate vinyl alcohol
(enol) that quickly tautomerizes to the more stable
ketone or aldehyde.

39.
Chapter 9 39
Mechanism of Mercuric Ion
Catalyzed Hydration
• The electrophilic addition of mercuric in (Hg+2
) creates
a vinyl carbocation.
• Water attacks the carbocation and after
deprotonation, forms an organomercurial alcohol.
• Hydrolysis of the alcohol removes the mercury,
forming a vinyl alcohol commonly referred to as enol.

40.
Chapter 9 40
Keto–Enol Tautomerism
• Enols are not stable and they isomerize to the
corresponding aldehyde or ketone in a
process known as keto-enol tautomerism.

41.
Chapter 9 41
Hydroboration–Oxidation
Reaction
• Alkynes can be hydrated anti-Markovnikov by using the
hydroboration–oxidation reaction.
• A hindered alkyl borane needs to be used to prevent two
molecules of borane to add to the triple bond. Disiamylborane
has two bulky alkyl groups.
• If a terminal alkyne is used, the borane will add to the least
substituted carbon.

42.
Chapter 9 42
Oxidation of Boranes
• In the second step of the hydroboration–oxidation, a
basic solution of peroxide is added to the vinyl borane
to oxidize the boron and replace it with a hydroxyl
group (OH).
• Once the enol is formed, it tautomerizes to the more
stable aldehyde.

44.
Chapter 9 44
Permanganate Oxidation of
Alkynes to Diketones
• Under neutral conditions, a dilute potassium
permanganate solution can oxidize a triple bond into
an diketone.
• The reaction uses aqueous KMnO4 to form a
tetrahydroxy intermediate, which loses two water
molecules to produce the diketone.

45.
Chapter 9 45
Permanganate Oxidation of
Alkynes to Carboxylic Acids
• If potassium permanganate is used under basic
conditions or if the solution is heated too much, an
oxidative cleavage will take place and two molecules
of carboxylic acids will be produced.